Radiation absorption test cell



July 28, 1970 M. BAUER RADIATION ABSORPTION TEST CELL Filed Aug. 25, 1965 II/AL I I I I I I 1 I I 1 I I! Fig.1

Morris Bader IN VEN TOR.

BY 2mm United States Patent 3,521,963 RADIATION ABSORPTION TEST CELL Morris Bader, 1402 Lorain Ave., Bethlehem, Pa. 18018 Filed Aug. 25, 1965, Ser. No. 482,513 Int. Cl. G011: 21/16, 1/10 US. Cl. 356-244 1 Claim ABSTRACT OF THE DISCLOSURE This invention relates generally to an improved cell and method of making cell windows therefor, which cell is utilized in analytical instruments employing infra-red radiation. More particularly, the present invention relates to the method of making infra-red transparent cell windows by a greatly simplified method than known heretofore and to provide a specific test cell structure for utilization with the windows so produced.

In the spectrophotometric analysis of organic liquids, fixed and demountable liquid cells are conventionally utilized to hold a sample of a liquid to be analyzed in the path of an infra-red beam. Such liquid cells generally utilized infra-red transparent windows which are produced from sodium chloride, for example by crystallizing pure molten sodium chloride to obtain an optical quality crystal window. Windows produced in this manner normally require grinding and polishing in order to prepare them for use in the test cells and additionally since solvents such as water, methanol and ethanol attack plates of this nature readily fogging them they must be frequently repolished, which polishing is time-consuming and costly.

The cost of an optical quality alkali metal halide crystal window produced commercially is many thousands of times the cost of the raw material, and as indicated the windows commonly produced from sodium chloride are highly vulnerable to moisture. While windows using materials other than sodium chloride are available, such windows are generally considerably more expensive than sodium chloride windows. However, while they do resist fogging they eventually cloud up and these windows must also be returned to the manufacturer for repolishing or a considerable amount of time and effort must be expended in the laboratory in an attempt to rejuvenate them through a repolishing operation.

As is suggested above, the largest single problem associated with the utilization of alkali metal halide windows is that they are severely attacked by water vapor inasmuch as they are water soluble to a degree and must generally be kept in a desiccater over silica gel in order to reduce the ambient humidity so as to prevent the fogging of the windows. Since such commercially produced alkali metal halide windows are relatively expensive the average laboratory must reuse a window for a great number of samples. This not only gives rise to a problem of maintaining the window in an unfogged condition such as might occur from the attack by solvents, such as water, methanol, ethanol, etc. but also gives rise to the problem of removing all traces of the previous sample as well as any traces of the solvents utilized to clean the windows. Obviously, after a window has been used a number of times impurity deposits surely exist. This problem is further compounded by the fact that the substances generally used for cleaning windows are generally detrimental to the infra-red absorption characteristics of the window.

Most infra-red spectrophotometric work in water solution is therefore limited because of the solubility of the window.

It is therefore an object of this invention to provide a greatly simplified method of producing infra-red transparent windows for spectrophotometer cells so inexpensively that the windows may be utilized for the analysis of a single sample and then discarded. Accordingly, by utilizing a disposable window of the nature described the attendant problems of solvent damage and contamination of the windows with infra-red absorptive agents are substantially reduced.

Another object of this invention is to provide an infra-red spectrophotometer test cell which facilitates the utilization of the windows produced in accordance with the method of this invention.

A further object of this invention is to provide an improved infra-red spectrophotometer test cell which is so designed to accept windows produced in accordance with the method of this invention regardless of such irregularities which may occur on the surface of the windows.

Still another object of this invention is to provide a method of making infra-red or transparent windows for spectrophotometric use which obviates the time-consuming and costly procedure of melting an alkali metal halide or the like and subsequently crystallizing the compound to produce an optically pure blank which must subsequently be polished to produce a usable infra-red transparent window.

Still another object of this invention is to provide a greatly simplified method of producing infra-red transparent windows for spectrophotometric analysis of water solutions of inorganic compounds and of compounds which normally attack such alkali metal halide windows.

A still further object of this invention is to provide a method of making infra-red spectrophotometer cell windows from alkali metal halides as well as other metallic halides such as from silver halides, thallium halides, and the like.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part 'hereof, wherein like numerals refer to like parts throughout, and in which:

FIG. 1 is an exploded perspective view of a spectrophotometer cell made in accordance with this invention and utilizing infra-red transparent windows produced in accordance with the method of the present invention; and

FIG. 2 is an enlarged transverse vertical sectional view of the device and windows of the cell of FIG. 1 as shown in their assembled relationship.

Briefly, the present invention provides a method of making infra-red transparent spectrophotometer windows not by melting and re-crystallizing but by merely pressing a finely particulate form of the material to be utilized for the window at high pressure and at room temperature. The windows are then utilized in a spectrophotometer test cell constructed in accordance with the present invention which cell is so designed to compensate for the surface irregularities sometimes encountered with windows produced in accordance with the method of this invention inasmuch as they do not undergo a planar surface generating or polishing operation.

An infra-red spectrophotometer test cell such as illustrated and described hereinafter may be utilized in conjunction with a spectrophotometer such as a Perkin-Elmer model 237 infra-red recording spectrophotometer.

With more particular regard to the method of the present invention I have discovered that with the aid of a hydraulic press such as a Harco Press it is possible to prepare highly desirable infra-red transparent spectrophotometer test cell windows from alkali metal halides such as potassium bromide, silver halides, thallium halides, and the like. Such a pressing operation normally requires the utilization of pressures in the range of approximately 15,000 to 30,000 pounds per square inch for approximately three to five minutes duration. Under such processing the halides fuse to a glassy state in which state the window so formed is quite transparent both in the visible and in the infra-red range. In addition, by appropriately selecting the starting halide utilized to prepare a window it is possible to match the index of refraction of the cell window to the index of refraction of the liquid sample being analyzed. Incidently, this practice is generally not possible when utilizing salt windows such as those commercially prepared by the melting and subsequent crystallization of a halide such as sodium chloride.

It will therefore be appreciated that infra-red spectrophotometer test cell windows produced in accordance with the novel method of this invention may be advantageously utilized for the spectrophotometric analysis of many materials which are deleterious to the windows inasmuch as the windows may be discarded after a single use without any great financial loss. In addition, windows prepared from a metallic halide such as silver chloride are extremely useful because of their total inertness to water and water vapor and because they are extremely transparent in the infra-red range from approximately 2 to microns.

EXAMPLE I A specific example of the method of producing infrared transparent spectrophotometer test cell windows in accordance with the present invention will now be discussed.

A suitable window may be prepared by triturating approximately 250 mg. of a spectrographic grade of an alkali metal halide such as potassium bromide. The triturated potassium bromide is placed in a pellet die such as utilized in a Harco Press of the type capable of subjecting the die to approximately 25,000 lbs. p.s.i. pressure or more.

When utilizing 250 mgs. of starting material the die should be selected so as to yield a finished window of approximately 13 mm. in diameter and approximately mm. to 1 mm. in thickness.

A conventional die of this nature is generally provided with a means of evacuating the intersticial air and accordingly the die loaded with the potassium bromide is evacuated to a pressure of approximately 1 or 2 mm. of mercury. The die is then compressed in a Harco Press to a pressure of approximately 20,000 lbs. p.s.i. which pressure is preferably maintaiend for approximately three minutes. During this period the potassium bromide fuses or coalesces into a self-sustaining window. Fusion of the pellet is sometimes accompanied by a slight decrease in the pressure indicated by the Harco Press. In the event the pressure does fall during the three minute holding period the pressure should then again be raised to approximately 20,000 lbs. p.s.i. and maintained at such pressure for at least an additional 30 seconds. At this time air may be bled into the vacuum line to the die and the hydraulic pressure on the Harco Press released and the die with the finished window therein removed.

The finished window, which as indicated above, requires no further polishing and will usually be perfectly transparent to the eye. However, windows are sometimes formed which have a slight opacity or whiteness and may 4 give the impression of being unusable, however, it is generally found that the windows are completely transparent in the infra-red range.

EXAMPLE II The method of Example I was repeated with the exception that a finely triturated metallic halide such as silver chloride 'was substituted for the potassium bromide and it was found that the fusion of the triturated silver chloride was effected by subjecting the die to a somewhat lower forming pressure of about 16,000 lbs. p.s.i. As indicated supra, silver chloride windows made in this manner are relatively inert and are sufficiently flexible to hold up over extremely long periods of time without any significant loss of windows from breakage. However, the windows may be produced so inexpensively that it is normally preferable to use a window for a single analysis therefore substantially obviating the possibility of erro neous results due to a contamination of the window from residual amounts of previous samples, window cleaning agents, etc.

EXAMPLE III The method of Example I was repeated with the exception that a finely triturated thallium halide such as thallium iodide or a mixture of thallium iodide and thallium bromide was substituted. The pressure required to fuse the thallium halide window is approximately 16,000 lbs. p.s.i. when maintained for approximately three minutes as set forth in Example I.

The infra-red transparent windows produced in accordance with Examples I, II, and III require the utilization of a special infra-red spectrophotometer test cell produced in accordance with this invention. An exemplary embodiment of which cell is illustrated in FIGS. 1 and 2, to which I 'will now specifically refer.

As seen in FIG. 1 the infra-red spectrophotometer test cell is indicated generally at 10 and in its assembled relation from the bottom to the top includes an annular base portion .12 provided with a central aperture 14 therethrough. The annular base 12 is further provided with a counterbore 16 concentric with the central bore 14 which cooperates with the central bore 14 to produce a stepped bore through the annular space member 12 thereby providing an upwardly facing annular shoulder 18 adapted to receive a generally flat annular gasket 20 formed of a suitable relatively rigid yet resilient material such as neoprene for example. The gasket 20 is sized so as to just fit in the counterbore 16 and has a central aperture substantially equivalent to the central bore 14. A relatively rigid washer 22 formed of steel for example and of the same dimension as the gasket 20 is then inserted into the counterbore 16 in contiguous relationship with the gasket 20.

An infra-red transparent window 24 produced in accordance with the method of the present invention and having a diameter substantially the same as the diameter of the counterbore 16 is then inserted in overlying contiguous relationship to the washer 22 and as seen best in FIG. 2 the window 24, which forms the bottom of the sample cell bridges across the central bore 14.

An annular spacer ring 26, preferably formed of a ductile metal such as lead for example, is then inserted into the counterbore 16 in overlying contiguous relationship to the bottom window 24. As seen best in FIG. 2 the thickness of spacer ring 26 determines the thickness of the film of liquid sample to be subjected to an infrared beam by virtue of the fact that a sample cell indicated generally at 28 is established when the top infrared transparent window 30 is placed on top of the spacer ring 26.

The gasket 20, washer 22, bottom window 24, spacer ring 26 and top window 30 are compressed in their assembled relationship shown in FIG. 2 against the annular upwardly facing shoulder 18 by means of a stepped pressure ring 32 having a central bore 15 therethrough which bore is complementary in diameter to the central bore 14 of the base 12. The pressure ring 32 is characterized by an upper annular flange 34, complementary in diameter to the annular base ring 12. As seen best in FIG. 2 the pressure ring 32 includes an integral depending annular portion 36 the outer diameter of which is complementary to the diameter of the counterbore 16 of the base ring 12.

The flange 34 of the pressure ring 32 is provided with a plurality of apertures equally spaced around the circumference thereof adjacent the periphery thereof such as at 38 which apertures 38 are adapted to be slidably received over a plurality of threaded studs 40 placed in a complementary position about the circumference of the top of the base ring 12 to provide a means of drawing down the pressure ring 32 to compress the gasket 20, washer 22, bottom window 24, spacer ring 26, and top window 30 in the assembled relationship shown in FIG. 2, which assembled relationship is of course established by securing the pressure ring 32 with a plurality of knurled nuts 42 complementary to the threaded studs 40 and threadably received thereon. The threaded studs 40 are preferably threadably received in suitable threaded apertures in the annular base ring 12, however, it will be understood that the threaded studs 40 may be secured by other means such as by press-fitting for example.

When assembling the infra-red transparent spectrophotometer test cell as shown in FIG. 2 the knurled nuts 42 will of course be drawn down in stepwise sequence so as to uniformly compress the various gaskets, spacers, windows, etc. so as to preclude damage to the Windows 24 and 30 and also permit any air trapped within the sample cell space 28 to be expelled as the pressure ring 32 is drawn down.

The test cell permits the utilization of the windows 24 and 30 produced in accordance with the method of the present invention inasmuch as the windows are sometimes characterized by slightly irregular surfaces inasmuch as they are not subjected to a polishing operation. Accord ingly, the utilization of test cells known heretofore in conjunction with the windows produced in accordance with the method of this invention is generally not possible due to such surface irregularities on the windows.

The infra-red spectrophotometer test cell 10 may be mounted in a spectrophotometer such as a PerkinElmer spectrophotometer by means of a suitable holder, not shown, which is adapted to interpose the test cell 10 with the annular bore 14-15 axially aligned with the infrared beam passing therethrough.

It may therefore be seen that there has been provided a greatly simplified method of producing infra-red transparent spectrophotometer test cell windows and a specific test cell structure which permits their utilization in a conventional infra-red spectrophotometer thereby fulfilling a need which has long existed for a relatively inexpensive means whereby infra-red transparent spectrophotometer windows may be produced and utilized in the analysis of both organic and inorganic materials.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention as claimed.

What is claimed as new is as follows:

1. A radiation absorption sample cell for holding a pair of radiation transparent windows made by subjecting a finely divided radiation transparent compound such as an alkali halide, silver halide, thallium halide and the like in a suitable forming die to a coalescing pressure in the range of approximately 16,000 to 20,000 lbs. psi. for a period of time in excess of 30 seconds, said cell comprising in successively assembled relation from bottom to top an annular window holding base, said window holding base having a central bore therethrough of lesser diameter than the windows to be supported and a concentric counterbore of greater diameter than the windows to be supported whereby an upwardly facing annular shoulder is defined in the central bore in said base, a generally fiat annular gasket to seat on said shoulder, a substantially rigid annular washer above said annular gasket, a bottom window to seat upon said washer above said annular gasket, a relatively rigid annular spacer ring to seat upon said bottom window, a top window to seat upon said ring, an annular compression member being provided with a lower annular face adapted to bear upon said upper window, the internal diameters of said annular gasket, washer, and spacer ring being substantially equal in diameter to the central bores of said base and said compression member, and means securing said base and said compression member in assembled relation, said securing means including a plurality of uniformly spaced threaded members secured to said base and extending upwardly therefrom, said compression member being provided with a plurality of apertures in alignment with said threaded members and a plurality of nuts threadably received on said threaded members to uniformly compress said top window downwardly toward said shoulder.

References Cited UNITED STATES PATENTS 5/1963 Haenni 356246 X 7/1968 De Grave et al 356-246 U.S. Cl. X.R. 

